Vinyl esters of higher carboxylic acids are of certain economic significance as comonomers. They can be used to modify the properties of polymers, for example polyvinyl chloride, polyvinyl acetate, polystyrene or polyacrylic esters. For example, the hydrolysis resistance of emulsion paints can be increased. For the production of adhesives too, vinyl esters of higher carboxylic acids are used. For these fields of use, vinyl esters based on 2-ethylhexanoic acid, isononanoic acid, lauric acid or the Versatic Acids 911, 10 and 1519 from Shell have been found to be of industrial significance. These higher carboxylic acids are obtainable, for example, by oxidation of aldehydes which have been prepared by the oxo reaction, or by the Koch synthesis from the olefin, carbon monoxide and water. In the case of vinyl esters based on 2-ethylhexanoic acid, lauric acid or isononanoic acid, if the isononanoic acid consists predominantly of 3,5,5-trimethylhexanoic acid, the compounds are homogeneous, whereas, in the case of vinyl esters of the Versatic Acids 911, mixtures of highly branched carboxylic acids having 9 to 11 carbon atoms are bound within the vinyl ester, and, in the case of vinyl esters of the Versatic Acids 1519, mixtures of highly branched carboxylic acids having 15 to 19 carbon atoms. In the case of vinyl esters of Versatic Acid 10, highly branched decanoic acids of differing structure, such as neodecanoic acids, are derivatized. 3,5,5-Trimethylhexanoic acid is prepared on the in scale by hydroformylation of diisobutylene and subsequent oxidation or the corresponding aldehyde (Ullmanns Encyklopädie der technischen Chemie, 4th edition, 1983, Verlag Chemie, Volume 9, pages 143-145; Volume 23, pages 608-607).
It is known that vinyl esters of carboxylic acids earn be prepared by reaction of acetylene with carboxylic acids (G. Hübner, Fette, Seifen, Anstrichmittel 68, 290 (1966) Ullmanns Encyklopädie der technischen Chemie, 4th edition, 1983; Verlag Chemie, Volume 23, pages 606-607). This procedure is adopted in EP 1 057 525 A2, according to which gaseous acetylene is reacted with the carboxylic acid to be vinylated in the presence of a catalyst in a tubular reactor. In the known multiphase process, the carboxylic acid comprising the catalyst, for example a zinc salt, in dissolved form constitutes the continuous phase in which gaseous acetylene is present as a dispersed phase. The tubular reactor is operated at a load factor of greater than 0.8. The use of acetylene as a raw material on the industrial scale, however, requires a high level of apparatus and safety complexity, and acetylene is additionally generally available only locally.
It is likewise known that the vinyl esters of carboxylic acids can be prepared by what is called the transvinylation reaction with a vinyl ester of another carboxylic acid;
where R and R1 may each be an aliphatic or aromatic radical. In order to control the equilibrium reaction in the direction of the products, a high excess of the transvinylating reagent R1—C(O)O—CH═CH2 is frequently used. The carboxylic acid R1—C(O)OH formed should also be of sufficient volatility to be withdrawn rapidly from the equilibrium and hence to generate an elevated conversion. Since the reaction mixture is generally worked up by distillation, the choice of transvinylating reagent R1—C(O)O—CH═CH2 is frequently guided by the boiling points of the other reaction participants (Ullmanns Encyklopädie der technischen Chemie, 4th edition, 1983, Verlag Chemie, Volume 23, pages 606-607). For the preparation of vinyl esters of higher carboxylic acids, vinyl acetate in particular and vinyl propionate to a certain extent are suitable as transvinylating reagents. Vinyl acetate, being a chemical produced on the industrial scale, is available inexpensively. Vinyl acetate and the acetic acid formed therefrom have comparatively low boiling points and can be separated by distillation from the desired vinyl ester of the higher carboxylic acid.
There are numerous hints in the literature regarding the transvinylation reaction of carboxylic acids with vinyl acetate as the vinylating reagent. Adelman, Journal of Organic Chemistry, 1949, 14, pages 1057-1077 studies the transvinylation of higher carboxylic acids, such as stearic acid, lauric acid or 3,5,5-trimethylhexanoic acid, with vinyl acetate in the presence of mercury salts as a catalyst. U.S. Pat. No. 2,245,131 discusses the reaction of benzoic acid or crotonic acid with vinyl acetate in the presence of mercury acetate. The reaction mixture is at first kept under reflux. Subsequently, the reaction temperature is increased and the acetic acid formed is removed. The vinyl benzoate formed is then purified in a further fractional distillation. U.S. Pat. No. 2,299,862 discloses the preparation of vinyl 2-ethylhexanoate proceeding from 2-ethylhexanoic acid and vinyl acetate in the presence of mercury acetate and sulphuric acid. The resulting crude mixture is first neutralized with sodium acetate and then distilled. Vinyl 2-ethylhexanoate is obtained with a yield of 73%. According to DE 1222493 B, the catalyst used for the transvinylation with vinyl acetate are mercury salts of a sulphonic acid cation exchange resin.
Disadvantages of the transvinylation processes with mercury catalysts are the toxicity and volatility thereof, and the unwanted formation of ethylidene diesters. The activation, typically with sulphuric acid, and the need to deactivate the catalyst prior to the distillation of the reaction mixture by neutralization also mean additional process steps.
These disadvantages can be avoided by the use of palladium catalysts, in the case of which the modification of the palladium complexes with aromatic nitrogen ligands, for example with 2,2′-bipyridyl or 1,10-phenanthroline, has been found to be advantageous. According to U.S. Pat. No. 5,214,172, the activity of palladium catalysts modified in this way is increased by the addition of strong acids. U.S. Pat. No. 5,741,925 discusses the transvinylation of naphthenic acids in the presence of palladium complexes modified with 2,2′-bipyridyl or 1,10-phenanthroline. In accordance with the known procedure, naphthenic acids, preferably cyclic C10-C20 carboxylic acids, are converted to the corresponding vinyl esters with vinyl acetate as the transvinylating reagent under reflux. The catalyst is stable during the distillation and can be reused in several runs. The process disclosed according to U.S. Pat. No. 5,223,621 relates to the transvinylation of carboxylic acids, for example of lauric acid or benzoic acid, with a (2,2′-bipyridyl)palladium(II) diacetate complex formed in situ under reflux. Prior to the distillation of the crude product, the catalyst is precipitated with oxalic acid or hydrochloric acid and filtered off.
The use of a combined catalyst system composed of a palladium salt and a redox agent for transvinylation of carboxylic acids is also known. EP 0 376 075 A2 recommends a redox-active catalyst system composed of palladium chloride, copper(II) bromide and lithium acetate. By way of example, the batchwise transvinylation of lauric acid with vinyl acetate close to the boiling point of vinyl acetate is described. The desired vinyl ester is obtained in pure form in a subsequent distillation. A further configuration of a redox-active catalyst system is disclosed in JP 2002-322125 A. This involves heating the reaction mixture composed of carboxylic acid and vinyl acetate, and also palladium acetate and lithium acetate, to 65° C.
Likewise mentioned in the prior art is the use of ruthenium catalysts for the transvinylation reaction. According to Murray, Catalysis Today 1992, 13, pages 93-102, higher carboxylic acids such as 2-ethylhexanoic acid, benzoic acid, neodecanoic acid, neononanoic acid or adipic acid can be converted to the corresponding vinyl esters with vinyl acetate in the presence of metallic ruthenium or ruthenium compounds such as ruthenium chloride, ruthenium oxide or ruthenium carbonyls such as Ru3(CO)12. This reaction is conducted batchwise under carbon monoxide or nitrogen at a pressure of about 2 bar and a temperature of typically 130 to 150° C. A corresponding process is likewise known from EP 0 351 603 A2 and EP 0 506 070 A2. It is pointed out that ruthenium catalysts are more thermally stable than palladium catalysts, which are deactivated at elevated temperatures with deposition of metallic palladium. However, in the case of the known ruthenium-catalysed processes, only moderate yields are reported.
The majority of the transvinylation reaction processes described in the prior art are conducted batchwise, usually under reflux and occasionally under pressure in a closed reaction vessel.
A continuously operated transvinylation process is known from EP 0 497 340 A2. By means of a continuously operated reactive distillation, by continuous removal of the most volatile reaction component, the equilibrium of the transvinylation reaction R—C(O)OH+R1—C(O)O—CH═CH2→R1—C(O)OH+R—C(O)O—CH═CH2 is shifted in the direction of the products. The transvinylating reagent R1—C(O)O—CH═CH2 is chosen such that the corresponding acid R1—C(O)OH is volatile and is removed from the equilibrium. The process according to EP 0 497 340 A2 uses preferably vinyl acetate as the transvinylating reagent, and the acetic acid formed is removed from the reaction zone together with unreacted vinyl acetate. Subsequently, the vinyl acetate separated from the acetic acid in a separate step is returned back to the reaction zone. The known process works with ruthenium catalysts, for example with [Ru(CO)2OAc]n, and describes the transvinylation of adipic acid, neodecanoic acid and 2-ethylhexanoic acid. In order, however, to suppress the unwanted formation of acid anhydrides, it is run only up to a partial conversion of the desired vinyl ester.
WO 2011/139360 A1 and WO 2011/139361A1 disclose a continuous and semicontinuous transvinylation process for carboxylic acids with vinyl acetate, using palladium complexes containing aromatic nitrogen ligands such as 2,2′-bipyridyl and 1,10-phenanthroline. The continuous process is operated in a bubble column with attached packed column, downstream of which may additionally be connected a rectification column and a stripping column. Vinyl acetate is introduced into the bubble column while, at the same time, a mixture of the carboxylic acid and vinyl acetate containing the catalyst in dissolved form is introduced into the attached packed column. Carboxylic acid and catalyst flow into the bubble column, while vinyl acetate is conducted through the bubble column and through the attached packed column in countercurrent. Vinyl acetate and acetic acid formed are removed and separated in a downstream rectification column and stripping column.
WO92/09554 A1 discloses a process for preparing vinyl esters in the presence of ruthenium catalysts, wherein the reaction mixture obtained is worked up using an azeotroping agent. The process can be performed continuously, semi-continuously, batchwise or semi-batchwise.
U.S. Pat. No. 4,425,277 discloses a continuous process for preparing vinyl esters. The reaction is effected in the presence of a supported palladium catalyst with an added cocatalyst composed of an alkali metal compound and a copper(II) compound.
EP 0 054 158 A1 likewise relates to a continuous process for preparing vinyl esters. The reaction is effected in the presence of a supported catalyst based on palladium(II) salts having activated carbon as a support material with a particular analytical SiO2 content.
A disadvantage of the continuous processes described in the prior art is that the transvinylating reagent vinyl acetate is not utilized efficiently for preparation of the vinyl ester because it vaporizes after a very short residence time due to the reaction conditions selected and is no longer available to the transvinylation reaction as a result. Moreover, the known continuously operated transvinylation processes are complex in terms of apparatus and have to be designed with high capital costs, since a series of columns have to be connected downstream of the reaction vessel, these working as additional reaction columns. The known processes therefore allow only moderate space-time yields of the desired vinyl ester.
There is therefore a need for a continuously operated process for transvinylation of carboxylic acids which is easy to perform having a low level of apparatus complexity and enables a high space-time yield, i.e. a high product output per unit reaction volume and time, of the desired vinyl ester. The desired vinyl ester is likewise to be obtained with a high selectivity.